The present invention relates to a piezoelectric actuator control apparatus, and more specifically to a piezoelectric actuator control apparatus adapted to driving piezoelectric actuators employed in a superfine positioning apparatus.
If a voltage is applied to a piezoelectric actuator to measure its extension, hysteresis is usually observed. The hysteresis, however, greatly decreases if the extension is measured with respect to the electric charge stored between electrodes of the piezoelectric actuator. The experimental results have been described in ELECTRONICS LETTERS, Vol. 18, No. 11, 1982, pp. 442-443.
FIG. 1 illustrates the circuit used for the experiments. A resistor 16 is connected in series with a piezoelectric actuator 14, and the other end of the resistor 16 is grounded. A voltage at a connection point between the piezoelectric actuator 14 and the resistor 16 is input to the negative input terminal of an operational amplifier 10, and a control signal Vin is input to the positive input terminal of the operational amplifier 10. The output of the operational amplifier 10 is input to a power circuit 12. The output of the power circuit 12 proportional to the output of the operational amplifier 10 is transmitted. The circuit of FIG. 1 is a typical constant-current circuit which so works that a constant current flows into the resistor 16.
In this circuit, a voltage applied to the resistor 16 is brought to be equal to the input voltage Vin. This means that a constant current flows into the resistor 16 and the electric charge is stored in the piezoelectric actuator 14 at a constant rate with respect to the time.
If the displacement is measured while the electric charge Q is being accumulated in the piezoelectric actuator 14 at a constant rate, there are obtained displacement characteristics that are nearly in proportional to the amount of electric charge. The above prior art, however, does not have a function to impart electric charge of a given amount to the piezoelectric actuator or to hold the electric charge of a given amount, in order to maintain constant the amount of displacement of the piezoelectric actuator. Summary of the Invention:
The object of the present invention is to provide a piezoelectric actuator control apparatus which controls the amount of electric charge imparted to the piezoelectric actuator in order to maintain constant the amount of displacement of the piezoelectric actuator.
In order to achieve the above object, the present invention deals with a piezoelectric actuator control apparatus which comprises a piezoelectric actuator having a material that produces strain upon application of an electric field and having electrodes for applying an electric field to said material, a capacitor connected in series with said piezoelectric actuator, and control means which applies a voltage to the piezoelectric actuator so that said capacitor will assume a predetermined voltage.
In general, the electrostatic capacity of the piezoelectric actuator does not remain constant for the voltage that is applied thereto. The electric charge Q involves a large hysteresis with respect to the applied voltage, and it is difficult to find the amount of electric charge Q directly from the applied voltage.
Therefore, a capacitor is connected in series with the piezoelectric actuator to store therein the electric charge Q of an amount equal to the electric charge Q induced in the piezoelectric actuator. Since the electrostatic capacity of the capacitor remains constant, the amount of electric charge Q of the capacitor is proportional to the voltage applied thereto without involving any hysteresis (FIG. 5).
More preferably according to the present invention, a capacitor is connected between one electrode of the piezoelectric actuator and a reference potential, provision is made of an operational amplifier which received a potential at the connection point between the capacitor and the piezoelectric actuator as well as a control signal, and which produces signal based upon a difference therebetween, and the potential of another electrode of the piezoelectric actuator is controlled based upon the signal produced from the operational amplifier.
A voltage Vin is input to the input terminal of the operational amplifier. Under the initial condition, the potential at a connection point between the capacitor and the piezoelectric actuator is equal to a reference potential. The electric charge of an equal amount is stored in the piezoelectric actuator and in the capacitor owing to the voltage produced by the operational amplifier. If the electrostatic capacity of the capacitor is denoted by Q, the operational amplifier so works that the electric charge Q=CVin will be stored.
Therefore, a given electric charge Q determined by Vin in imparted to the piezoelectric actuator and is held therein. FIG. 6 shows displacement characteristics. The input voltage Vin and the electric charge Q establish a proportional relationship Q=CVin without involving hysteresis.
According to the present invention, furthermore, an integrating circuit is connected in series with one electrode of the piezoelectric actuator, and the potential of the other electrode of the piezoelectric actuator is so controlled that the integrating circuit will produce an output of a predetermined value.
The integrating circuit is provided to integrate a current i that flows into the piezoelectric actuator. The amount of electric charge Q is detected from Q =it (t denotes the time). The voltage applied to the piezoelectric actuator is controlled by the electric charge Q that is detected.
This makes it possible to control the piezoelectric actuator relying upon the amount of electric charge. That is, the electric charge of a given amount is imparted to the piezoelectric actuator and is held therein.